Syntactic foam

ABSTRACT

A low density syntactic foam comprising micro-spheres bound by a cured liquid phase binder is formed by allowing the micro-spheres ( 1 ) to rise to a close packed surface layer ( 4 ) in the liquid phase binder ( 2 ), and then draining excess liquid phase binder from the mould ( 3 ) before curing. The use of the buoyant force to achieve close packing of the micro-spheres is enhanced by controlling the viscous drag and curing time in the liquid phase binder ( 2 ) by selecting predetermined amounts of diluent, which is typically acetone in an epoxy resin binder.

The present invention relates to a method of manufacturing low densitysyntactic foam.

BACKGROUND OF THE INVENTION

Syntactic foam is made up of a mix of pre-formed micro-spheres and abinder, typically an epoxy resin. Syntactic foams have been used inareas where low densities are required with high strength as inundersea/marine equipment for deep-ocean current metering,anti-submarine warfare and sandwich composites.

The process of manufacturing syntactic foams is different from that ofconventional foams. In one known manufacturing process, theconsolidation method for binder and micro-spheres includes the coatingof micro-spheres, prior to which are the steps of vacuum filtering andrinsing. Other manufacturing processes make use of inorganic bindersolution and firing, dry resin powder for sintering, and liquid resin asbinder for in-situ reaction injection moulding.

A major difficulty is always to achieve a sufficiently low density. Onemanufacturing process for syntactic foam has been developed using acompaction method which includes a mixture of liquid resin/micro-spheresachieving a resin volume fraction of 0.09 and a density of 0.6 g/cc.

A slip casting method employing porous plaster moulds to drain excessiveliquid binder has also been developed. An advantage of this method isits potentially suitability for the manufacture of thick items while thedisadvantages could be the limited service life of the plaster mould andpoor surface finish.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

DISCLOSURE OF THE INVENTION

According to the present invention there is provided a method ofmanufacturing syntactic foam including the steps of:

-   -   providing a predetermined ratio of constituent materials        including a liquid phase binder and microspheres that are        naturally buoyant in that binder;    -   blending the constituent materials into a mixture and placing        the mixture into a mould;    -   allowing the microspheres to float to the top of the mixture;    -   draining excess liquid phase binder from the mould; and    -   allowing the remaining liquid phase binder to set or cure        between the micro spheres.

Preferably the microspheres are allowed to float to the top of themixture until they become close packed.

Preferably the microspheres become close packed in a density approachingthe natural microsphere bulk density.

Preferably the step of allowing the microspheres to float to the top ofthe mixture until they become close packed is facilitated by selecting aliquid phase binder composition that has sufficiently low viscous dragcharacteristics, and sufficiently long curing time, to allow themicrospheres to become close packed before the binder cures.

Preferably the liquid phase binder composition is selected by adding apredetermined amount of diluent.

Preferably the liquid phase binder includes an epoxy resin withhardener, and the diluent comprises acetone.

Preferably the excess liquid phase binder is drained from the bottom ofthe mould.

Preferably the liquid phase binder is drained until the close packedmicrospheres reach the bottom of the mould.

BRIEF DESCRIPTION OF THE DRAWINGS

Not withstanding any other forms that may fall within its scope, onepreferred embodiment of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 a) is a diagrammatic view of a mixture of micro-spheres andliquid phase binder immediately after being placed into a mould;

FIG. 1 b) is a similar view to FIG. 1 a) after a period of time suchthat the micro-spheres are partially packed due to the buoyancy of themicro-spheres in the liquid binder;

FIG. 1 c) is a similar view to FIG. 1 b) after a further period of timesuch that the micro-spheres are closely packed; and

FIG. 1 d) is a similar view to FIG. 1 c) after an additional furtherperiod showing the closely packed layer of micro-spheres gravitated downthe mould after excess liquid phase binder is drained.

PREFERRED EMBODIMENTS OF THE INVENTION

In the laboratory, a method of manufacturing syntactic foam typicallyincludes the steps of, mixing a liquid phase binder by first placing aclean, empty mixing container on an electronic scale, adding apredetermined amount of acetone, and then injecting pre-mixed epoxy andhardener into the container using a plastic syringe until the requiredmass is reached. The container is closed and shaken vigorously for 2minutes. The container is then opened and the micro-spheres addedthrough a glass funnel. The container is then sealed and shaken againvigorously for a further 5 minutes to disperse the micro-spheres.

For subsequent casting, the container is kept shaken to maintain aconsistent mixture ratio whilst the mixture is being poured through atube into a mould.

Referring to FIG. 1(a), a mixture of micro-spheres 1 and liquid phasebinder 2 are placed into a mould 3 after thorough mixing such that themicro-spheres 1 are dispersed evenly within the liquid phase binder 2.

The micro-spheres are chosen to be naturally buoyant in the liquid phasebinder and are typically whole hollow spheres. The ultimate performanceof the syntactic foam can be improved by carefully selecting appropriatewhole hollow micro-spheres and conducting the mixing process in a mannerwhich reduces the number of broken spheres to a minimum.

The micro-spheres 1 which are initially in suspension with the liquidphase binder 2 as shown in FIG. 1(a) begin by the natural buoyancyeffect to move to the upper surface of the mould as shown in FIG. 1(b).

Over time, the micro-spheres 1 begin to self-pack into a close packedlayer 4 as shown in FIG. 1(c) by the natural effect of the buoyant forceexerted on each micro-sphere immersed in the liquid phase binder 2.

Over time, the separation becomes more complete with the close packedmicro-spheres clustered on the surface of the binder which can then bedrained from the bottom of the mould through an opening 6 as shown inFIG. 1(d). The excess liquid phase binder is typically drained to thepoint where the close packed layer of micro-spheres touches the bottomof the mould forming a layer 5 at the bottom of the mould as can beclearly seen in FIG. 1(d).

The process can take different periods of time depending on the natureof the liquid phase binder, but typically after 30 minutes in alaboratory situation, the foam in the mould is sufficiently dry and theliquid phase binder cured so that the layer 5 can be de-moulded.

Although the process has been described in a laboratory situation, itwill be apparent that it can be adapted to a production situation andthat syntactic foams of different shapes and configurations can be madedepending on the shape of the mould 3 and the amount of micro-spheresintroduced in the original mixture.

To utilise the method according to the invention efficiently, it isimportant to select the characteristics of the liquid phase binder suchthat the micro-spheres have sufficient time to rise into the closepacked layer 4 before the binder cures or thickens sufficiently that theviscous drag will inhibit the close packing of the micro-spheres. Thisis typically achieved by adding selected amounts of diluent to thebinder. In the case of an epoxy resin binder, this diluent is typicallyacetone as described above.

The ratio of acetone to epoxy resin and hardener is important as addingtoo much acetone will not only significantly defer the curing time ofthe resin, but will also affect the buoyant force on the micro-spheresdue to the lower density of the liquid phase binder. The effect of theseratios can be seen in the example given below.

Densities of manufactured syntactic foam with various amounts ofconstituent materials are listed in FIG. 2 Table 1. From Table 1, we seethe measured bulk density. Different mixing ratios were employed byvarying acetone content but keeping the mass ratio of micro-spheres to(epoxy+hardener) constant as 1 to 2 as seen in the second column of FIG.2 Table 1. As can be seen from Table 1, the foam density decreases asthe acetone content increases, and very closely approaches the bulkdensity of the micro-spheres which are the lower limit of the achievablefoam density. In general, since the density of liquid phase is muchhigher than that of micro-spheres, a low foam density can be achieved byreducing the amount of liquid phase or increasing packing density ofmicro-spheres. The higher the buoyant force, the higher the packing isexpected unless there are other factors contributing to the packing.

While the buoyant force may be a main driving force in forming thecurrent syntactic foam, factors affecting the buoyant force may includeviscosity and densities of constituent materials. The net buoyant forcemay be reduced in the presence of a viscous drag such thatNBF (net buoyant force)=BF (buoyant force)−viscous drag.

The viscous drag increases with increasing viscosity. However, as theacetone content increases, the viscous drag decreases due to thedecrease in viscosity of liquid mixture as shown. Concurrently, the BFalso decreases because of the decrease in density of liquid phase. Thus,the improvement in lowering the foam density by the addition of acetoneappears to be due more to decrease in the viscous drag than that in theBF, which results in increased NBF.

Thus a new manufacturing method using a BF technique has been developedfor syntactic foam. It has been demonstrated that the syntactic foamdensity can be lowered down to 0.08 which is very close to itsmicro-sphere bulk density of 0.72.

1. A method of manufacturing syntactic foam including the steps of:providing a predetermined ratio of constituent materials including aliquid phase binder and microspheres that are naturally buoyant in thatbinder; blending the constituent materials into a mixture and placingthe mixture into a mould; allowing the microspheres to float to the topof the mixture; draining excess liquid phase binder from the mould; andallowing the remaining liquid phase binder to set or cure between themicrospheres.
 2. A method as claimed in claim 1 wherein the microspheresare allowed to float to the top of the mixture until they become closepacked.
 3. A method as claimed in claim 2 or wherein the microspheresbecome close packed in a density approaching the natural microspherebulk density.
 4. A method as claimed in claim 2 wherein the step ofallowing the microspheres to float to the top of the mixture until theybecome close packed is facilitated by selecting a liquid phase bindercomposition that has sufficiently low viscous drag characteristics, andsufficiently long curing time, to allow the microspheres to become closepacked before the binder cures.
 5. A method as claimed in claim 4wherein the liquid phase binder composition is selected by adding apredetermined amount of diluent.
 6. A method as claimed in claim 5wherein the liquid phase binder includes an expoxy resin with hardener,and the diluent comprises acetone.
 7. A method as claimed in claim 1wherein the excess liquid phase binder is drained from the bottom of themould.
 8. A method as claimed in claim 2 wherein the liquid phase binderis drained from the bottom of the mould until the close packedmicrospheres reach the bottom of the mould.
 9. A syntactic foam articlecomprising close packed microspheres bound together by a cured,originally liquid phase binder, manufactured by a method according toclaim 1.